1. Field of the Invention
The present invention relates to a defect detection circuit for an optical pick up device.
2. Description of the Related Art
An optical pick up device is disclosed in Japanese Patent Application Laying Open (KOKAI) No.63-155426 wherein a sample servo control system is used for controlling the focusing and tracking operation of the device. According to the disclosed system, the focusing control and the tracking control of the optical spot irradiated on the optical information recording medium is conducted in such a way that an error amount of a new sample is compared with an amount of preceding sample and if the difference between the two amounts is less than a predetermined value, the new sample amount is adopted as a valid amount for the new sample, whereas if the difference is more than the predetermined value, an amount for the new sample is determined by adding the predetermined value to the preceding sample amount.
Such a control method is called window method. According to the window method, the defects in the prepit portion can be detected and it becomes possible to avoid malfunction of the servo system due to the sampling error caused by defective pits formed on the recording medium, which reduces the noise due to the sampling error.
In general, there are two methods for detecting a defective pit. A first method is a pattern matching method in which a defective pit is detected on the basis that the refresh signal from the prepit portion has a constant pattern so that the pit having a pattern other than the constant pattern is identified as being defective.
A second method is a window method, which is used in the above-mentioned patent document, wherein if the difference between the amounts of the adjacent pits exceeds a predetermined value, the pit is identified as being defective.
With regard to the first pattern matching method, it is inconvenient to set a threshold value for digitizing the refresh signal at the time of identifying the pattern of the signal.
Also, with regard to the second window method, the defective pit is detected on the basis of the relative value between the adjacent pits. It is hard to accurately and reliably identify the defective pit only from such a relative value.